In a wide variety of clinical and therapeutic situations, blood platelet counts are routinely needed. Abnormalities in platelet counts can cause significant bleeding problems in a patient, and may indicate a multitude of underlying conditions. Measurement of blood platelet counts typically requires complex instrumentation and a clinical laboratory. Automated hematology analyzers can be used to obtain platelet counts over a wide range of values; however, manual hemacytometer counts are necessary for measurement of low platelet counts. Other blood constituents, such as red and white blood cells, as well as instrument artifacts may interfere with accurate assessment of platelet count. Methods for determination of platelet count by a simple, portable procedure are necessary.